In an electrochemical gas sensor, the gas to be measured typically diffuses from the atmosphere into the sensor housing through a gas porous or gas permeable membrane to a working electrode (sometimes called a sensing electrode) where a chemical reaction occurs. A complementary chemical reaction occurs at a second electrode known as a counter electrode (or an auxiliary electrode). The electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte gas (that is, the gas to be detected) at the working and counter electrodes.
To be useful as an electrochemical sensor, a working and counter electrode combination must be capable of producing an electrical signal that is (1) related to the concentration of the analyte and (2) sufficiently strong to provide a signal-to-noise ratio suitable to distinguish between concentration levels of the analyte over the entire range of interest. In other words, the current flow between the working electrode and the counter electrode must be measurably proportional to the concentration of the analyte gas over the concentration range of interest.
In addition to a working electrode and a counter electrode, an electrochemical sensor often includes a third electrode, commonly referred to as a reference electrode. A reference electrode is used to maintain the working electrode at a known voltage or potential. The reference electrode should be physically and chemically stable in the electrolyte and carry the lowest possible current to maintain a constant potential.
Electrical connection between the working electrode and the counter electrode is maintained through an electrolyte. The primary functions of the electrolyte are: (1) to efficiently carry the ionic current; (2) to solubilize the analyte gas; (3) to support both the counter and the working electrode reactions; and (4) to form a stable reference potential with the reference electrode. The primary criteria for an electrolyte include the following: (1) electrochemical inertness; (2) ionic conductivity; (3) chemical inertness; (4) temperature stability; (5) low cost; (6) low toxicity; (7) low flammability; and (8) appropriate viscosity.
Electrochemical gas sensors of the type discussed above are generally disclosed and described in U.S. Pat. Nos. 4,132,616, 4,324,632, 4,474,648; and in European Patent Application No. 0 496 527 A1. A comprehensive discussion of electrochemical gas sensors is also provided in a paper by Cao, Z. and Stetter, J. R., entitled "Amperometric Gas Sensors," the disclosure of which is incorporated herein by reference.
In general, the electrodes of an electrochemical cell provide a surface at which an oxidation or a reduction reaction occurs to provide a mechanism whereby the ionic conduction of the electrolyte solution is coupled with the electron conduction of the electrode to provide a complete circuit for a current. It is generally believed that the half cell reactions of the working electrode and the counter electrode, respectively, for nitrogen dioxide (NO.sub.2) electrochemical gas sensors (using H.sub.2 SO.sub.4 as the electrolyte) are as follows: EQU NO.sub.2 +2H.sup.+ +2e.sup.- .revreaction.NO+H.sub.2 O EQU H.sub.2 O.revreaction.1/2O.sub.2 +2H.sup.+ +2e.sup.-
The above reactions result in the following net cell reaction: EQU NO.sub.2 .revreaction.NO+1/2O.sub.2
The measurable current arising from the above cell reaction is directly proportional to the rate of reaction. Preferably, therefore, a high reaction rate is maintained in the electrochemical cell. For this reason, the counter electrode and/or the working electrode of the electrochemical cell generally comprise an appropriate electrocatalyst on the surface thereof to enhance the reaction rate. If the reaction rate of either half cell reaction is impeded, resulting in a low exchange current density, the equilibrium current of the electrochemical cell may be easily perturbed during measurement. Such deviation can result in undesirable side reactions and/or nonlinear behavior over the range of nitrogen dioxide concentrations desired to be detected.
The phrase "exchange current density" as used in connection with the present invention refers generally to the normalized exchange current or exchange current per unit area. The exchange current is generally defined as the level of balanced faradaic activity or net chemical change occurring at an electrode when the net current is zero. The exchange current is proportional to the native rate at which either, or in the ideal case, both, the anodic and cathodic reactions occur for a given chemical species at an electrode. The lower the exchange current density, the more sluggish is the electrode reaction, and the more difficult it is to obtain useful output from the system. On the other hand, the larger the exchange current, the faster the electrode reaction occurs. At high exchange currents, large currents can be supplied by the system with ease. "The exchange current can be viewed as a kind of `idle speed` for charge exchange across the electrode-electrolyte interface." Bard, A. J. and Faulkner, L. R., Electrochemical Methods, Fundamentals and Applications, John Wiley & Sons, New York, pp. 100-107 (1980).
To achieve adequate exchange current density in existing electrochemical sensors for detecting nitrogen dioxide, the counter electrode generally comprises an electrocatalyst such as, for example, platinum (Pt) or iridium (Ir), suitable to catalyze the oxidation reaction occurring at that electrode. Such an electrochemical sensor is presently available, for example, from City Technology of Portsmouth, England. In the City Technology sensor (that is, the Nitrogen Dioxide CiTicel), the working and reference electrodes are fabricated from carbon, while the counter electrode comprises a Pt electrocatalyst.
As somewhat evident from the above discussion, the type, rate, and efficiency of the chemical reactions within an electrochemical gas sensor are controlled, in significant part, by the material(s) used to make the working electrode and counter electrode. Indeed, extensive research efforts are expended to develop improved working electrodes, counter electrodes and electrochemical systems generally. See Cao, supra, at 49. As part of these efforts, manufacturers of electrochemical sensors continuously attempt to simplify the manufacturing process and to reduce the costs involved therein, while maintaining suitable sensor performance specifications.
It is desirable, therefore, to develop new electrodes and electrode combinations for use in electrochemical gas sensors for the detection of nitrogen dioxide which achieve the above-referenced goals.